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PHYSIOLOGICAL 385 



Along with the fats are classed the lipoids or fat-like substances, 

 of which the most important is lecithin, which occurs, along with 

 the protein vitellin, in the yolk of many different kinds of eggs. 

 Its formula begins like that of the fats, but ends quite differently 

 with an atom of phosphorus and a nitrogen-containing base. There 

 are other members of this class which become wrapped up with 

 sugars. Of the importance of these and other substances in the 

 chemical routine (or metabolism) of the bod}^ account will be taken 

 in the section on "Everyday Functions"; we are here concerned 

 simply with a survey of cell materials. 



So far, then, in this stocktaking of cell materials, we have 

 recognised : — 



(i) the proteins and their component amino-acids; and the 

 compounds that proteins form, especially with nucleic acid; 



(2) the carbohydrates, such as sugars and starch ; 



(3) the fats, with their associated lipoids, and 



(4) mineral salts, iron, and so forth. 



Along with these organic compounds and abundant water 

 (80 per cent.), there are always to be found mineral salts, such as 

 potassium chloride. And there are also traces of other elements, 

 especially iron, which have to be fitted in eventually into the 

 cellular framework. But here, if our simple general perspective is to 

 be maintained, must end our survey of the chemical materials of 

 the cell. 



Staining. — It may now be asked with reason whether any- 

 thing is known of the combination of dyes with the constituents 

 of the cell in fixed and stained microscopical preparations. It 

 must be frankly admitted that the answer is disappointing; we 

 know too little of the nature of these combinations, and our ideas 

 of the location of various substances within the cell is very vague. 

 The dyes generally employed in microscopy fall into two great 

 classes, acid dyes and basic dyes. But it must be realised that in 

 fact most of the dyes are neutral salts, and the term "acid dye" 

 means a dye with an organic acid and a mineral base. The basic 

 dyes, which have the opposite chemical structure, are divided into 

 those which (like methylene blue and gentian violet) stain the 

 fixed tissues directly, and those (like hsematoxylin) which may 

 require a mordant solution to prepare the way before them; they 

 combine with the nucleus, especially with the nucleolus or the 

 chromosomes— in fact, with chromatin. On various grounds the 

 conclusion is arrived at that chromatin or nucleo-protein is a salt 

 of basic protein and nucleic acid; this acid it is which combines 

 with the basic dye, and its chemical nature, although complex, is 

 so far known; it appears to contain phosphoric acid, sugars, and 

 nitrogenous bases; there seems to be one sort of nucleic acid for 



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